Tunable Electrochemical and Catalytic Features of BIAN- and BIAO-Derived Ruthenium Complexes

This article deals with a class of
ruthenium–BIAN-derived complexes, [RuII(tpm)­(R-BIAN)­Cl]­ClO4 (tpm = tris­(1-pyrazolyl)­methane, R-BIAN = bis­(arylimino)­acenaphthene,
R = 4-OMe ([1a]­ClO4), 4-F ([1b]­ClO4), 4-Cl ([1c]­ClO4), 4-NO2 ([1d]­ClO4)) and [RuII(tpm)­(OMe-BIAN)­H2O]2+ ([3a]­(ClO4)2). The R-BIAN framework with R = H, however, leads to the selective
formation of partially hydrolyzed BIAO ([N-(phenyl)­imino]­acenapthenone)-derived
complex [RuII(tpm)­(BIAO)­Cl]­ClO4 ([2]­ClO4). The redox-sensitive bond parameters involving
NC–CN or NC–CO
of BIAN or BIAO in the crystals of representative [1a]­ClO4, [3a]­(PF6)2,
or [2]­ClO4 establish its unreduced form. The
chloro derivatives 1a+–1d+ and 2+ exhibit one oxidation
and successive reduction processes in CH3CN within the
potential limit of ±2.0 V versus SCE, and the redox potentials
follow the order 1a+ < 1b+ < 1c+ < 1d+ ≈ 2+. The electronic structural
aspects of 1an–1dn and 2n (n = +2, +1, 0, −1, −2,
−3) have been assessed by UV–vis and EPR spectroelectrochemistry,
DFT-calculated MO compositions, and Mulliken spin density distributions
in paramagnetic intermediate states which reveal metal-based (RuII → RuIII) oxidation and primarily BIAN-
or BIAO-based successive reduction processes. The aqua complex 3a2+ undergoes two proton-coupled redox processes
at 0.56 and 0.85 V versus SCE in phosphate buffer (pH 7) corresponding
to {RuII–H2O}/{RuIII–OH}
and {RuIII–OH}/{RuIVO}, respectively.
The chloro (1a+–1d+) and aqua (3a2+) derivatives are
found to be equally active in functioning as efficient precatalysts
toward the epoxidation of a wide variety of alkenes in the presence
of PhI­(OAc)2 as oxidant in CH2Cl2 at 298 K, though the analogous 2+ remains
virtually inactive. The detailed experimental analysis with the representative
precatalyst 1a+ suggests the involvement of
the active {RuIVO} species in the catalytic cycle,
and the reaction proceeds through the radical mechanism, as also
supported by the DFT calculations.